Sensors for the detection of particular compounds present in a high temperature gas stream find numerous applications in many different mechanical systems. For example, detection of certain compounds in a high temperature gas stream is important in industrial emission monitoring for detection of gas pollutants, such as sulfur dioxide (SO.sub.2), in residential heating systems for detection of carbon monoxide (CO), and in automobile exhaust systems for various compounds including hydrocarbons.
In automotive applications, gas sensors can be placed at various locations in an exhaust system. Exhaust gas from an internal combustion engine typically contains hydrogen (H.sub.2), carbon monoxide (CO), methane (CH.sub.4), carbon dioxide (CO.sub.2), oxides of nitrogen (NO.sub.x), water (H.sub.2 O), and non-methane hydrocarbons (C.sub.n H.sub.m), where n is an integer larger than 1 and m is an integer whose value depends upon the kind of hydrocarbon compound, for example, alkane, alkene, alkyl, or aryl. Important environmental pollution concerns dictate that the emission of hydrocarbons be minimized. To minimize pollutants in the engine exhaust, sensors can be placed before and after the catalytic converter to monitor the performance of the converter. Also, the emission of hydrocarbons can be controlled, in part, by an engine exhaust control system that receives a feedback signal from an exhaust sensor capable of selectively detecting the presence of hydrocarbons in the engine exhaust.
Several types of sensing elements have been developed for detecting various chemical species within a gas stream. These sensing elements includes calorimetric sensors having a catalyst coating, or a semiconductor metal oxide, or the like. Calorimetric hydrocarbon gas sensors measure the amount of heat released by the catalytic oxidation of hydrocarbons contained within the exhaust gas. To obtain optimum sensitivity for the measurement of hydrocarbon species within a gas stream, a calorimetric hydrocarbon gas sensor must be designed to maintain a relatively constant internal temperature, and the flow of gas within the sensor must be carefully regulated. This requirement is especially important given the wide temperature variations encountered in an industrial or automotive gas system.
In addition to temperature regulation requirements, precise gas concentration measurements require that a reference catalyst be provided in close proximity to a measurement catalyst. The reference catalyst is used to compensate for environmentally induced temperature responses in the microcalorimeters, and to provide a means for selectively measuring the concentration of one particular chemical species within the gas stream. In order for the dual catalyst system to function properly, the gas flow conditions must be identically maintained in regions proximate to each catalyst. This requires that the temperature, flow velocity, and turbulence conditions of the gas in proximity to each catalyst be substantially the same. The maintenance of precisely controlled gas flows within a gas sensor becomes especially important when the sensor is deployed within a high velocity gas stream, such as an automotive exhaust system, or an industrial gas expulsion system, or the like. Accordingly, a need existed for an apparatus capable of precisely regulating the temperature and flow conditions of a gas within a gas sensor.